In a spot-type disc brake one brake shoe is actuated directly by the piston of a hydraulic actuating apparatus. In this configuration, the piston is in abutment against the backplate of the brake shoe and urges the brake shoe axially against the brake disc during the braking operation. It is state of the art to rivet a retaining spring to the backplate of the brake shoe. The retaining spring is typically shaped as a spring plate having spring arms that engage the hollow piston and abut against its inside wall. By means of the spring, the brake shoe is captivated to the piston so that upon braking it is entrained by the piston when the latter is being retracted and a clearance is thereby created between the brake shoe and the brake disc.
Prior-art retaining springs are comprised of a plane, central fixing section which is in abutment against the backplate of the brake shoe and which is formed with an opening for the passage of a rivet. Laterally said fixing section passes over to three spring arms which are bent off substantially at right angle relative to the plane of the fixing section and which jut out from the backplate in order to extend into the hollow brake piston. In this context, two spring arms are disposed opposite each other with respect to the axis of the piston and the third one at right angle to it.
It is, moreover, prior art to furnish the backplate with a buttonhead through tab so that a projection standing out from the plane of the backplate is formed to which the retaining spring is fixed.
Furthermore, it is state of the art to coat the backplates with a silencing varnish paint in order to prevent braking noise. The silencing varnish paint is customarily applied by means of a roller being passed over the backplate. However, no projections must then jut out from the plane of the backplate which could damage said roller.
It is an object of the invention to create a brake shoe in which a retaining spring is fixed to a projecting buttonhead through tab of the backplate and in which the backplate is coated with silencing varnish paint.
The arrangement of the present invention bears the advantage that the silencing varnish paint can be applied in the proven manner by means of a roller. Said roller requires only a slight modification by dividing it into two sections or part-rollers in order to avoid the projecting buttonhead through tabs. In this way a band-shaped range of the backplate will result which is free of silencing varnish paint and in which the buttonhead through tabs are positioned.
In order not to impair the effect of the silencing varnish paint, the band-shaped range must be as narrow as possible. Because of its width, a plane fixing section of the retaining spring would abut against the backplate both in the band-shaped range and in the range coated with silencing varnish paint and become more or less deformed during the riveting depending on the thickness of the layer of silencing varnish paint. An undefined deformation of the fixing section leads, however, to an unpredictable position of the spring arms, so that their full operativity can no longer be relied on.
Because of the presence of the plateau-shaped step, the installation of the fixing section on the backplate is limited to the section of the backplate which is free of silencing varnish paint, whereby any deformation is avoided and the operability of the spring arms, in particular a constant force of pressing against the brake piston, is, advantageously, preserved.
On account of the inventive reduced abutment surface of the fixing section, a retaining spring which is fastened in the customary manner by one single buttonhead through tab having a circular cross section will be safeguard against twisting.